Precious metals such as gold and silver have good electrical conductivity and chemical stability, and thus they or their alloys can be used as contact materials in the switches, but their expensive prices limit their application scope. Base metals such as iron, cobalt, nickel, aluminum, copper, titanium, etc., as well as their alloy, including stainless steel, have good chemical stability in the atmospheric conditions, but their electrical conductivity when used as contact materials is usually less ideal than precious metals, their conductivity is usually less than gold and silver, and their chemical stability is poorer than precious metals such as gold and silver. Furthermore, their electrical wear resistance is generally worse than gold, silver and other precious metals. In comparison with precious metals, base metals are usually cheap, readily available and in abundant supply. As conductive materials or as contact materials, they are much less expensive than precious metals. In short, independent use of any of metals results in both advantage and disadvantages.
Precious metal plating (particularly gold plating) on contacts made from stainless steel and other base metal substrates is an effective method to improve contact performance. Technical personnel have made a lot of efforts to improve gold plating methods and the gold plating quality of the contacts. For Example, Chinese Patent Application No. 201310564337 “Process Method of Gold Plating on Mini-sized Relay Contacts and Reeds” discloses the solutions to instability of contacting resistance and statistic bonding function by using specific plating fluid formulas and improving the density and purity of the gold plating. USA Patent Application 20140045352 “Connector with Gold-palladium Plated Contacts” discloses a method for controlling the color of the binary metal alloy by adjusting the gold and palladium concentration in the binary metal alloy. Japanese Kokai Tokkyo Koho 2003-057111 discloses a technique to eliminate the pinholes in the plating and improve the corrosion resistance of silver-based contacts, by the use of a sputtering method to coat a 1 μm thick gold layer, and then by the use of electroplating method to coat a 1 μm thick gold layer on the silver-based contacts. The so-prepared contacts have good reliability even in highly corrosive gas environments.
Chinese Patent Application No. 201010557154.2 “Fabrication Process of Partial Gold Plating Sheets” and a series of patent documents filed by the same applicant and applied on the same date disclose a method to implement partial gold plating, wherein a protective dry membrane is photo-exposed selectively, so as to shelter the non-plating zone. An alkaline-dissolved protective dry membrane and an exposure machine must be used in this method. Chinese Patent Application No. 200910023466.2 “A Spare Part Laminating Method before Partial Gold Plating on Reeds” discloses a method of one-side gold plating, also using an exposure machine. U.S. Pat. No. 4,077,852 “Selective Gold Plating” discloses selectively electroplating gold on metallic surfaces containing copper using a chromate film as a plating mask. This procedure permits reduced use of gold without adversely affecting device performance. In addition, chromate films may be patterned with relatively high dimensional resolution so as to achieve gold plating patterns useful in electronic and integrated circuits. The chromate film is made by the cathodic deposition of potassium dichromate. However, when chromate is used as a plating mask, the gold electroplating conditions must be milder than the commonly used gold plating condition. Lower plating bath temperatures and lower voltage or current density, for example, must be used for the plating.
The Applicant's Chinese Patent Application No. 201310748955 “A Switch Contact Component and Its Preparation Method” discloses a three-layer laminated switch contact component: the substrate is rubber, the inter layer is a continuous base metal sheet, and the upper layer is discontinuous precious metal plating, or discontinuous binary metal composite layer of base metal plating and precious metal plating. The structure, which has good electric conductivity and stability, dust-resistance and oil stain-resistance when used as a switch contact component, is formed by printing a partial plating mask and plating. However, the contact component made by this preparation method has limited plating thickness. The plating thickness is restricted by the thickness of the plating mask printed thereon. As we know, it is difficult to obtain a very thick printed ink layer. The thickness of printed inks is usually no more than twenty microns. Therefore, the thickness of plating made by this method is less than twenty microns.
The insufficient thickness means the height of the conducting contact surface of the convex dots protruding out of the substrate is not enough, and therefore, the contact's dust resistance and oil stain resistance shall be impacted. When the plating thickness exceeds the thickness of the plating mask, the plating mask will be submersed in the plating. When submersed in the plating, the plating mask, which is electrically insulated, may spill to the contact surface and lead to an increase of contact resistance, even to a failed conductivity function. In addition, the contact angle of the printed ink on the base metal substrate is difficult to be controlled to 90°. That is, it is very difficult to make the edge of the printed ink perpendicular to the surface of the base metal substrate, and thus the edge of the plating is hardly perpendicular to the base metal substrate.
Steel, stainless steel, copper or copper alloy, nickel or nickel alloy are common base metals for the preparation of contacts. For the etching on these metals, etching solutions containing relatively strong inorganic acids such as HCl+HNO3+FeCl3+caprolactam (used for stainless steel etching) (Chinese Patent Application No. 201010160309.9), HNO3+FeCl3+NaCl (used for stainless steel etching) (Chinese Patent Application No. 201310100019.9), fluoboric acid or fluosilicic acid+methyl sulfonic acid or amino sulfonic acid+water soluble ferrous salt (used for the etching of tin without corrosion to copper and nickel) (Chinese Patent Application No. 201310187160.7), nitric acid or sulfuric acid+hydrogen peroxide+specific polymers (China Patent Application No. 200910023466.2), nitric acid and/or sulfuric acid+hydrogen peroxide+ammonium+aromatic amine+nitro compounds (Chinese Patent Application No. 201110110116.7), nitric acid+nickel nitrate+iodic acid+amino acid (U.S. Pat. No. 4,556,449), sulfuric acid+peroxide+low molecular weight carboxylic acid (used for copper etching) (U.S. Pat. No. 4,462,861), sulfuric acid+nitrate+hydrogen peroxide (JP2004-52001), Ferric chloride+hydrochloric acid+high molecular compound (JP2000-336491), phosphoric acid+hydrogen peroxide (JP2006-294797), and perchloric acid+ceric ammonium nitrate (JP2004-59973), are used as etching solutions. Unless otherwise mentioned, the etching solutions disclosed in these patent documents are all used for the etching of nickel or nickel alloys.
Etching solutions containing organic acids were disclosed in some patent documents. Chinese Patent Application No. 201080059307.9 “Etching Solution and Semiconductor Device Manufacturing Method Using It” discloses an etching solution containing hydrogen peroxide, organic acids (citric acid and malic acid) and a small amount of phosphonic acid, which can selectively etch copper without etching nickel. Chinese Patent Application No. 200610151609.4 discloses that the etching solution mainly based on acetic acid and containing acetic acid stabilizers, organic acids, inorganic acids and salts, wherein the salts were used to control the pH value of the etching solution and to adjust the relative etching rates of copper and molybdenum.
Although these patent documents discloses a wide variety of etching solutions, it is necessary to develop new etching solutions, by which iron, cobalt, nickel, copper and their alloys are etched and good etching results, such as glossy etched metal surfaces, no side etching and no corrosion to gold, silver and other precious metal, are achieved.